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Description and causes

Most organisms that reproduce sexually have pairs of chromosomes in each cell, with one chromosome inherited from each parent. In such organisms, a process called meiosis creates cells called gametes (eggs or sperm) that have only one set of chromosomes. The number of chromosomes is different for different species. Human beings have 46 chromosomes (i.e. 23 pairs of chromosomes). Human gametes have only 23 chromosomes.

If the chromosome pairs fail to separate properly during cell division the egg or sperm may have a second copy of one of the chromosomes. (Seenon-disjunction.) If such a gamete results in fertilization and an embryo, the resulting embryo may also have an entire copy of the extra chromosome.

Terminology

Trisomies are sometimes characterised as "Autosomal trisomies" (trisomies of the non-sex chromosomes) and "Sex-chromosome trisomies." Autosomal trisomies are described by referencing the specific chromosome that has an extra copy.[How to reference and link to summary or text] Thus, for example, the presence of an extra chromosome 21, which is found in Down syndrome, is called trisomy 21.

Human trisomy

Trisomies can occur with any chromosome, but often result in miscarriage. For example, Trisomy 16 is the most common trisomy in humans, occurring in more than 1% of pregnancies. This condition, however, usually results in spontaneous miscarriage in the first trimester.

The most common types of autosomal trisomy that survive to birth in humans are:

The most common forms of autosomal trisomy are trisomy of chromosome 21 which results in Down syndrome and trisomy of chromosome 18 which results in Edwards syndrome. In rare cases, a fetus with trisomy of chromosome 13 can survive, giving rise to Patau syndrome. Autosomal trisomy can be associated with birth defects, mental retardation and shortened life expectancy.